CH3CH2CH2CH3 Lewis structure, Polar or nonpolar, Isomers, Acid or base, Intermolecular forces

The IUPAC name for the structure represented by CH₃CH₂CH₂CH₃ is butane.

The word butane is a combination of two words i.e., but and ane. ‘But’ means four. As there are 4 carbon atoms in CH₃CH₂CH₂CH₃ so ‘alk’ in alkane is replaced with the prefix ‘but’ and it gives us the IUPAC name ‘but-ane’ or butane.

The chemical compound is usually represented by n-butane where n signifies that it is a straight-chain alkane. The alphabet n is also sometimes used to represent normal butane because CH₃CH₂CH₂CH₃ is the most dominant isomer of butane. It is 99% present in a mixture of butane isomers.

Other names for butane that are rarely used are Diethyl Methylethylmethane and tetracarbane.

CH₃CH₂CH₂CH₃ structure

The condensed structural formula CH₃CH₂CH₂CH₃ can be expanded to obtain the structure of n-butane, as shown below.

The terminal carbon atoms are bonded to 1 C-atom and 3 H-atoms via single covalent bonds. The middle carbon atoms are bonded to 2 C-atoms and 2 H-atoms, each.

In this way, each C-atom forms a total of 4 covalent bonds which represents a complete octet electronic configuration. Conversely, each H-atom in the CH₃CH₂CH₂CH₃ structure forms only 1 covalent bond and achieves a complete duplet configuration.

There are a total of 13 single covalent bonds (sigma bonds) in the above structure.

CH₃CH₂CH₂CH₃ Lewis structure

The Lewis structure of a molecule is a simplified representation of all the valence electrons present in it.

The Lewis structure of n-butane is shown below. Each straight line represents a single covalent bond containing 2 electrons. Thus, there are a total of 13(2) = 26 valence electrons in the Lewis structure of CH₃CH₂CH₂CH₃.

CH₃CH₂CH₂CH₃ intermolecular forces and physicochemical properties

There is a strong intramolecular covalent bonding between C-C and C-H atoms in the butane molecule. However, the intermolecular forces of attraction between individual CH₃CH₂CH₂CH₃ molecules are weak.

There is a negligible electronegativity difference between a C-atom and an H-atom while no electronegativity difference exists at all between two identical C atoms. Thus both C-C and C-H bonds are non-polar in the CH₃CH₂CH₂CH₃ molecules. Consequently, no permanent dipoles develop in the molecule neither a strong force of attraction exists between the molecules.

Temporary (instantaneous) dipoles develop due to the continuous motion of electrons around molecular nuclei. This results in London dispersion forces, the weakest intermolecular force of attraction between CH₃CH₂CH₂CH₃ molecules.

The boiling point of CH₃CH₂CH₂CH₃

It is due to the weak intermolecular forces of attraction that n-butane has an extremely low boiling point (-0.5°C) and a high vapor pressure (170 kPa) at room temperature.

CH₃CH₂CH₂CH₃ attractive forces can be easily overcome. Therefore, n-butane exists as a gas that quickly vaporizes and is highly liquefiable at the same time.

Solubility of CH₃CH₂CH₂CH₃

Water (H₂O) molecules can break the temporary attractive forces between CH₃CH₂CH₂CH₃ molecules to a small extent therefore n-butane is slightly water-soluble.  However only 61 mg n-butane dissolves per liter of water, this is because like dissolves like.

Water (H₂O) is a polar molecule while n-butane is non-polar so water cannot completely dissolve CH₃CH₂CH₂CH₃ molecules. Rather, n-butane is highly soluble in organic solvents of like polarity such as n-hexane, ethanol, etc.

Similarly, n-butane can dissolve non-polar compounds in itself such as lipids, oils, and waxes. 

Is CH₃CH₂CH₂CH₃ polar or non-polar?

As already discussed, n-butane is a non-polar molecule. According to Pauling’s electronegativity scale, a chemical covalent bond is considered polar if the bonded atoms have an electronegativity difference between 0.5 to 1.6 units.

However, in CH₃CH₂CH₂CH₃ no electronegativity difference exists between the bonded C atoms in a C-C bond.

A small electronegativity difference of 0.35 units exists between a carbon (E.N = 2.55) and a hydrogen (E.N = 2.20) atom in a C-H bond in CH₃CH₂CH₂CH₃. 0.35 < 0.5 so the C-H bond is also only weakly polar.

But the small dipole moments (symbol µ) of C-H bonds eventually get canceled in the symmetrical structure of CH₃CH₂CH₂CH₃. The electron cloud stays uniformly distributed in the molecule overall thus n-butane is non-polar (net µ = 0).

Is CH₃CH₂CH₂CH₃ acid or base?

n-butane (CH₃CH₂CH₂CH₃) is a saturated hydrocarbon which means there are strong sigma bonds between the bonded C-C and C-H atoms in the continuous straight chain structure.

Consequently, no covalent bond can readily break nor can a new bond be formed easily. As the C-H bond cannot break easily so n-butane is clearly not a proton donor as expected of a Bronsted-Lowry acid.

Also, there are no delocalized electrons present on any atom in the CH₃CH₂CH₂CH₃ molecule. As per Lewis acid-base concept, acids are electron pair acceptors while bases are electron pair donors. This means the n-butane molecule is neither acidic nor basic.

Butane (CH₃CH₂CH₂CH₃) isomers

Isomers are chemical compounds that have the same molecular formula but a different structural formula.

Other than the straight chain structure of n-butane, C₄H₁₀ can also be represented by a branched structure, as shown below. This is called iso-butane as it is an isomer of the normal butane. Its IUPAC name is 2-methyl propane.

n-butane and iso-butane are constitutional isomers.

Butane also exhibits another type of isomerism known as conformational isomerism.

Conformational isomers also called conformers are stereoisomers i.e., they have the same molecular formula, and the same structural formula in 2 dimensions but their 3-dimensional arrangement in space differs.

Conformers can be interconverted just by the rotation of a single bond, without breaking and/or making a new bond.

There are 4 conformational isomers of butane, as shown below. You may notice that these conformers are cyclic structures unlike aliphatic isomers of butane. Therefore, these are called cyclobutanes.

FAQ

Summary

• CH₃CH₂CH₂CH₃ is the condensed structural formula for n-butane.
• n-butane is the naturally abundant, straight chain isomer of butane (molecular formula = C₄H₁₀, molar mass = 58.122 g/mol).
• CH₃CH₂CH₂CH₃ exists as a colorless gas with a gasoline-like odor at r.t.p.
• Strong single covalent bonds exist between C-C and C-H bonded atoms in CH₃CH₂CH₂CH₃.
• Weak intermolecular forces of attraction are present between CH₃CH₂CH₂CH₃ molecules therefore it has a low boiling point (-0.5°C).
• CH₃CH₂CH₂CH₃ is a non-polar molecule.
• It is only slightly water-soluble but highly miscible with other non-polar organic compounds.
• Iso-butane is the branched isomer of butane.